Compensation of Volumetric Errors in a Gas Monitoring System

ABSTRACT

A mainstream gas monitoring system and method that includes a using a mainstream airway adapter, and a gas sensing assembly associated with the mainstream airway adapter to measure an analyte of a gas flow through the adapter. A gas sensing portion outputs a signal indicative of the analyte in a gas flow in the mainstream airway adapter. A processing portion receives the signal from the gas sensing portion and determines an amount of the analyte in the gas flow based on the signal from the gas sensing portion. The processing portion also compensates for volumetric differences between the gas flow during inspiration and the gas flow during expiration to maximize the accuracy in the measurements made using such a system and method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) fromprovisional U.S. patent application Nos. 60/957,272, filed Aug. 22,2007, and 60/873,047, filed Dec. 4, 2006, the contents of each of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a system and method for compensatingfor volumetric changes that may occur in a mainstream gas monitoringsystem between inspiration and expiration to (a) maximize the accuracyof an oxygen consumption measurement and (b) provide a more clinicallyrelevant waveform for the monitored gas when using a mainstream gasmonitoring system.

2. Description of the Related Art

Airway oxygen monitoring, or oxygraphy, is used in anesthesia andcritical care situations to provide an indication of oxygen delivery toand utilization (i.e. oxygen consumption) by the patient. The differencebetween inspired and end-tidal oxygen fraction is useful to determine,for example, the amount of oxygen extraction which serves as a measureof cardiac and pulmonary function (e.g. adequacy of perfusion andmetabolism) and overall physiologic condition of the patient. Oxygenconsumption is commonly used to monitor the fitness or physiologicalcondition of an individual or athlete. The phrases “oxygen update” and“oxygen consumption” are used synonymously, and are both represented bythe expression “{dot over (V)}_(O) ₂ ” or, for simplicity “VO₂”. Oxygenconsumption is a measure of the amount of oxygen that the body uses in agiven period of time, such as one minute. It is typically expressed asmilliliters of oxygen used per kilogram of body weight per minute(ml/kg/min), simply in milliliters of oxygen used per minute.

Traditionally, oxygraphy is accomplished using a side-stream gassampling system. In a sidestream monitoring system, a gas sample istaken from a sample site, such as the patient's airway via a nasalcannula or a patient circuit through a gas sampling line, to a sensingmechanism or sample cell that is located some distance from the samplesite for monitoring. A drying system is typically included in thecannula, sample cell, or sampling line so that sidestream flow of gasentering the sample cell is relatively moisture free. If the dryingsystem consists of a section of Nafion tubing, the gas sample is driedto ambient humidity. Similarly, the transport of the gas through thesampling line results in the temperature of the sample equilibrating tothe ambient temperature prior to analysis by the sensor. For thesereasons, both inspired and expired gas, are analyzed in side-streammonitoring systems as if the gas was at ambient temperature andhumidity.

When oxygraphy is performed using an on-airway oxygen sensor (i.e., amainstream gas sensor in which all or most the gas delivered to orreceived from the patient passes through the sample site), the gas beinganalyzed will likely vary in both temperature and humidity. The expiredgas is nearly always 100% saturated (relative humidity=100%) and at bodytemperature or slightly below body temperature. On the other hand, theinspired gas may be actively heated and humidified using a vaporizer,may be passively humidified using a heat-moisture exchanger, or may beat ambient conditions. In any case, it is unlikely that the intra-airwaytemperature and humidity data will be available to the oxygen monitor.

As gas is humidified and water vapor is added to the gas, the oxygen inthe gas is diluted and the concentration of oxygen in the gas decreases.If inspired gas is dry and expired gas is humidified, the oxygraphmeasured by an on-airway (mainstream) oxygen sensor will show adifference in inspired and expired oxygen just based on the changes ofoxygen concentration due to warming and/or humidification. While theoxygen concentration measurement is physically accurate, it isclinically misleading. Even though there is an actual difference ininspired and expired oxygen fraction, this difference could bemisinterpreted as an indication of patient perfusion and metabolism,rather than simple gas warming and humidification.

VO₂ is conventionally calculated as the difference between the volume ofoxygen inspired and the volume of oxygen expired. The standard or directcalculation of VO₂ is given by the following equation:

VO ₂ =Vi*Fi _(O2) −Ve*Fē _(O2),  (1)

where: “VO₂” is oxygen consumption, “Vi” is inspired volume, “Fi_(O2)”is the inspired oxygen concentration, “Ve” is the expired volume, and“Fē_(O2)” is the mixed expired oxygen concentration. An error occurs inthis calculation if the expired gas has been heated and moistened by thelungs, and the inspired gas is cooler and/or drier than the expired gas.The effect of heating and/or humidification means that the expiredvolume (Ve) will be larger than the inspired volume (Vi) and themeasured expired oxygen fraction (Fē_(O2)) will be lower than the actualoxygen fraction, leading to a falsely large VO₂ determination. Ideally,if it were possible to measure the inspired oxygen fraction (Fi_(O2))and inspired volume (Vi) under known temperature and humidityconditions, then the direct VO₂ calculation would be accurate despitethe differences in temperature and humidity between inspired and expiredgasses.

This direct calculation of oxygen consumption described in equation (1)is simple and valid, but it can lead to errors in the calculated VO₂ insituations where there are small errors in the gas volume measurement,i.e., the measurement of Vi, Ve, or both. Gas temperature and humiditydifferences are a major source of these inspired-expired gas volumedifferences. This problem is exacerbated at high oxygen concentrations.

An alternative method of calculating VO₂ uses only the expired breathvolume, Ve. In this scenario, the inspired breath volume Vi iscalculated (rather than measured) based on the assumption that, onaverage, the nitrogen volume is the same for both inspired and expiredgas, which is usually true because nitrogen is not consumed or producedby the body. This is referred to as the nitrogen balance. Thecalculation of Vi, rather than measuring it, also assumes that theeffect of temperature and humidity are the same for both inspired andexpired gas volumes.

This modification of equation (1), which uses a calculation of Vi basedon the nitrogen balance noted above, is known as the Haldane transform.According to this technique, Vi is calculated as follows:

Vi=Ve*Fē _(N2) /Fi _(N2)  (2)

Where “Fē_(N2)” is the concentration of mixed expired nitrogen, and“Fi_(N2)” is the concentration of mixed inspired nitrogen. The nitrogenconcentration can be calculated as the balance gas (or gas that isneither oxygen or CO₂, both of which are directly measured) assumingthat the only gases in the airways are oxygen, carbon dioxide, andnitrogen. Based on this, the Haldane transform becomes:

Vi=Ve*(1−Fē _(CO2) −Fē _(O2))/(1−Fi _(CO2) −Fi _(O2)),  (3)

and the oxygen consumption calculation becomes:

VO ₂ =Ve*[Fi _(O2)*((1−Fē _(CO2) −Fē _(O2))/(1−Fi _(CO2) −Fi _(O2)))−Fē_(O2)],  (4)

where Fē_(CO2) is the mixed expired carbon dioxide concentration, andFi_(CO2) is the mixed inspired carbon dioxide concentration.

Calculating VO₂ using the Haldane transform has the advantage that theeffects of errors in volume measurements that are not “common mode” areeliminated, because only the expired volume measurement is used. Commonmode errors are errors that affect both the Vi and Ve measurements, suchas a calibration error in a flow sensor. Assuming, of course, the samesensor is used to measure Ve and Vi.

As noted above, expired volume is often larger than inspired volumebecause the exhaled gas is warmed and humidified by the lungs. When theHaldane transform is used, the added volume due to temperature andhumidity causes an invalid estimation of Vi when the Fi_(O2) is measuredin dry gas, which leads to an erroneously high calculated Vi and VO₂.

This is typically not a problem when a conventional side-stream gassampling system is used to measure Fi_(O2), Fē_(O2), Fi_(CO2), andFē_(CO2), because conventional side-stream gas sampling systemstypically include a gas drying system, as noted above.

If, however, a mainstream monitoring system is used to measure FiO₂,Fē_(O2), Fi_(CO2), and Fē_(CO2), the use of the inspired-expiredtemperature and humidity differences can lead to an error. In amainstream monitoring system, the sampling site is located in-situ in apatient circuit or conduit coupled to the patient's airway. As a result,the patient's expired gases are normally saturated with water vapor andhave a temperature of about 35° C. If the inspired gas is cooler and/ordrier than the expired gas, errors result in the VO₂ calculation usingeither the direct or Haldane transform equations. Using the directcalculation method, it would be necessary to correct both the inspiredoxygen fraction (Fi_(O2)) and the inspired volume (Vi) to expired gastemperature and humidity conditions to make a correct calculation. Usingthe Haldane transform, only the inspired oxygen fraction must becorrected to the same warm and wet conditions as seen in expired gas forthe calculation to be valid.

SUMMARY OF THE INVENTION

The present inventors recognized that oxygraphy data acquired from anon-airway sensor would be more useful clinically, if the signal could bemodified such that inspired and expired data is displayed as if bothwere at the same conditions of temperature and humidity. To this endthey developed a means for correcting the oxygraphy waveform such thatinspired and expired portions of the waveform are displayed as if bothwere at the same conditions by compensating for volumetric measurementsof oxygen without the use of intra-airway temperature and humiditymeasurements.

Accordingly, it is an object of the present invention to provide a gasmonitoring system that overcomes the shortcomings of conventional gasmonitoring system and that can be used as a mainstream gas monitor. Thisobject is achieved according to one embodiment of the present inventionby providing a mainstream gas monitoring system that includes amainstream airway adapter, and a gas sensing assembly associated withthe mainstream airway adapter. The gas sensing assembly includes a gassensing portion associated with the mainstream airway adapter and aprocessing portion. The gas sensing portion is configured and arrangedto output a signal indicative of an analyte in a gas flow in themainstream airway adapter. The processing portion receives the signalfrom the gas sensing portion and determines an amount of the analyte inthe gas flow based on the signal from the gas sensing portion. Theprocessing portion also compensates for volumetric differences betweenthe gas flow during inspiration and the gas flow during expiration.

It is yet another object of the present invention to provide a method ofmonitoring system an analyte in a gas flow using a mainstream gasmonitoring system that does not suffer from the disadvantages associatedwith conventional analyte monitoring techniques. This object is achievedby providing a method that includes (1) providing mainstream airwayadapter having a gas flow therethrough; (2) producing a signalindicative of an analyte in the gas flow; (3) determining an amount ofthe analyte in the gas flow based on the signal from the gas sensingportion; (4) compensating for volumetric differences between the gasflow during inspiration and the gas flow during expiration; and (5)providing a signal indicative of the amount of analyte in a humanperceivable format.

These and other objects, features, and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention. As usedin the specification and in the claims, the singular form of “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first embodiment of a gas sensingsystem according to the principles of the present invention;

FIG. 2 is a perspective view of an airway adapter and gas sensor in thegas sensing system of FIG. 1;

FIG. 3 is a schematic view of the components of the gas sensing systemof FIG. 1;

FIG. 4 is schematic view of the components of a second embodiment of agas sensing system according to the principles of the present invention;

FIG. 5 is a perspective view of an airway adapter and gas sensoraccording to a third embodiment of the present invention;

FIG. 6 is a perspective view of an airway adapter and gas sensoraccording to a still further embodiment of the present invention;

FIG. 7 is a chart showing the relationship between the partial pressureof CO2 and O2 during exhalation;

FIG. 8 is a graph showing an exemplary oxygen percent waveform and anexemplary flow waveform over a single breath illustrating how the oxygenconcentration measurement changes due to gas heating and/orhumidification; and

FIG. 9 is a graph showing two exemplary oxygen percent waveforms createdby mechanically ventilating a test lung with dry room air and addinghumidification to the gas exiting the test lung.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 schematically illustrates an exemplary embodiment of a mainstreamgas monitoring system 30 according to the principles of the presentinvention. Gas monitoring system 30 includes an airway adapter 32 and agas sensing assembly, generally indicated at 34. Airway adapter 32 isdisposed in a respiratory circuit 40, which is used to communicate aflow of gas to a patient. For example, a first end 42 of respiratorycircuit 40 includes a patient interface appliance configured tocommunicate with an airway of a patient. Examples of patient interfaceappliances that are suitable for use with respiratory circuit 40include, but are not limited to: an endotracheal tube, a nasal cannula,a tracheotomy tube, a mask, or any other device or apparatus thatcommunicates a flow of gas with an airway of a user.

A second end 44 of respiratory circuit 40 is configured to communicatewith a gas source. For instance, the gas source may include ambientatmosphere, a supply of pressurized gas, a pressure support device, aventilator, or other sources of gas. In the illustrated embodiment, aY-connector 46, which is typically found in a ventilator circuit, isshown connected to the second end of the airway adapter. One leg of theY-connector corresponds to the inspiratory limb, which delivers gas froma ventilator (not shown) to the patient, and the other leg of theY-connector corresponds to the expiratory limb, which delivers gas fromthe patient. Typically, the gas is delivered by the expiratory limb backto the ventilator, which is the gas source in this embodiment. In asingle limb system, a single conduit communicates a flow of gas betweenthe patient and the gas source, which is often a pressure supportsystem, such as a CPAP, bi-level, or auto-titrating pressure supportdevice.

As perhaps best shown in FIGS. 2 and 3, airway adapter 32 provides aflow path 50 in-line with respiratory circuit 40 through which gaspasses to and from the patient. The airway adapter also provides a gasmonitoring portion or sample site, generally indicated at 52, at whichthe constituents of the gas passing through the airway adapter aremonitored or measured. Examples of airway adapter suitable for use inthe present invention are described in U.S. Pat. Nos. 5,789,660 (“the'660 patent”) and 6,312,389 (“the '389 patent”), and in U.S. patentapplication Ser. No. 09/841,451 (publication no. 2002/0029003)(“the '451application”), the contents of each of which are incorporated herein byreference.

In the embodiment illustrated in FIGS. 1-3, gas sensing assembly 34includes a gas sensing portion 36 and a processing portion 38. In thisillustrated exemplary embodiment, gas sensing portion 36 is removablycoupled to airway adapter 32, as indicated by arrow A, and includes thecomponents that are used to detect the gas constituent or constituents,also referred to as analyte, being monitored. It should be appreciatedthat a variety of mechanisms may be implemented to removably couple gassensing portion 36 to airway adapter 32. In an exemplary embodimentshown in FIG. 2, a seating area 33 is provided on an outer surface ofairway adapter 32 that is adapted to securely receive a housing 37 ofgas sensor portion 36. Housing 37 is generally “U” shaped to fit ontoseating area 33 with a channel 35 that receives the generally matchingshape of the seating area of the airway adapter. Flanges 39 can beprovided on the airway adapter to align and attach the housing to theairway adapter. U.S. Pat. Nos. 6,616,896 (“the '896 patent”) and6,632,402 (“the '402 patent”), the contents of each of which areincorporated hereby by reference, describe techniques for coupling gassensing portion 36 to airway adapter 32. The present invention alsocontemplates permanently connecting gas sensing portion 36 to airwayadapter 32 so that the functionality of each component is effectivelycombined into a common element.

A communication link 48 allows data, power, and any other signals,commands, etc. to be communicated between gas sensing portion 36 andprocessing portion 38. Although a hard wired communication link 48 isshown in FIGS. 1-3, it is to be understood that the present inventioncontemplates that the communication link can be a wireless link, usingany form of wireless communication or communication protocol. Of course,if a wireless link is provided, a power supply, such as battery, must beincluded in gas sensing portion 36 or a power must be provided in someother manner to the gas sensing portion.

Gas sensing assembly 34 detects the concentration of one or more gases(analytes) in the flow of gas through the sample cell. In an exemplaryembodiment illustrated in FIGS. 1-3, gas sensing assembly 34 isconfigured to employ luminescence quenching techniques to measure thepartial pressure or amount of oxygen or other gases that flow throughairway adapter 32. This oxygen measurement is used (in conjunction withflow), for example, to determine the values for inspired and mixedexpired fractions of oxygen (Fi_(O2) and Fē_(O2)).

Luminescence quenching is a technique that has been used to measureoxygen concentrations in gases. In using luminescence quenching tomeasure oxygen concentrations, a luminescable material 60 is excited toluminescence by delivering an excitation energy, as indicated by arrowB, to the luminescable material. Upon being excited to luminescence, theluminescable material will emit energy, as indicated by arrow C.However, when the luminescing material is exposed to a gas mixtureincluding oxygen, the luminescence is quenched and the luminescablematerial will emit less energy, as indicated by arrow C, depending uponthe amount (i.e., concentration or fraction) of oxygen to which theluminescable material is exposed, or the amount of oxygen in the gasmixture. Accordingly, the rate of decrease in the amount ofluminescence, or quenching of luminescence, of the luminescable material(i.e., the amount of light emitted by the luminescable material)corresponds to the amount of oxygen in the gas mixture. Thus, the energyemitted by the luminescable material can be used to determine theconcentration of the gas passing through the airway adapter. U.S. Pat.Nos. 6,325,978; 6,632,402; 6,616,896; and 6,815,211, the contents ofeach of which are incorporated herein by reference, all disclose anexample of an oxygen sensor that uses luminescence quenching todetermine the concentration of a gas, such as oxygen, in the gas flowingthrough a sample cell.

As shown in FIGS. 1-3, a quantity of luminescable material 60 issituated such that it is exposed to the gas flowing in flow path 50through airway adapter 32. The present invention also contemplatesproviding a combination of luminescable materials in communication withthe gas flowing through the airway adapter. Porphyrins are an example ofa material that may be used as luminescable material 60. Porphyrins arestable organic ring structures that often include a metal atom. When themetal atom is platinum or palladium, the phosphorescence decay timeranges from about 10 μs to about 1,000 μs. Porphyrins are also sensitiveto molecular oxygen. When porphyrins are used as luminescable material60, it is preferred that the porphyrins retain substantially all oftheir photo-excitability with repeated use. Stated another way, it ispreferred that the porphyrins be “photostable”. Fluorescent porphyrins,such as meso-tetraphenyl porphines, are particularly photostable. Thevarious types of porphyrins that may be used as luminescable material232 to facilitate oxygen detection include, without limitation, platinummeso-tetra(pentafluoro)phenyl porphine, platinum meso-tetraphenylporphine, palladium meso-tetra(pentafluoro)phenyl porphine, andpalladium meso-tetraphenyl porphine. Of course, other types ofluminescable materials that are known to be quenched upon being exposedto oxygen, carbon dioxide, or another analyzed substance (e.g., gas,liquid, or vapor) may also be used in airway adapters incorporatingteachings of the present invention.

In the illustrated embodiment, luminescable material 60 is provided onairway adapter 32, and a window 62 is provided in an opening 64 in thebody of the airway adapter to allow excitation energy B to betransmitted to the luminescable material. Window 62 preferably has ahigh transmittance for wavelengths of excitation radiation, which exciteluminescable material 60, as well as for wavelengths of radiation Cemitted from luminescable material. For example, window 62 may be formedof sapphire, one or more polymers (e.g., polyethelyne, etc.), a glass,and/or other substantially transparent materials.

In an exemplary embodiment, luminescable material 60 is carried by amembrane or matrix, which is disposed on or comprises an integral partof a surface or wall of the airway adapter defining gas flow path 50.The present invention also contemplates that the luminescable materialand associated components, such as a membrane, need not be directlycoupled to the airway adapter, but can be selectively coupled so thatthe luminescable material can be replaced without having to remove orreplace the entire airway adapter.

An emitter 66 is provided in gas sensing portion 36 to emit excitationenergy B to luminescable material 60. In an exemplary embodiment of thepresent invention, the energy emitted by emitter 66 includeselectromagnetic radiation with a wavelength that causes luminescablemedium 60 to luminensce. Emitter 66 may include one or more OrganicLight Emitting Diodes (“OLEDs”), lasers (e.g., diode lasers or otherlaser sources), Light Emitting Diodes (“LEDs”), Hot Cathode FluorescentLamps (“HCFLs”), Cold Cathode Fluorescent Lamps (“CCFLs”), incandescentlamps, halogen bulbs, received ambient light, and/or otherelectromagnetic radiation sources.

In one exemplary implementation, emitter 66 includes one or more greenand/or blue LEDs. These LEDs typically have high intensity in theluminescable composition absorption region of luminescable medium 60 andoutput smaller amounts of radiation at other wavelengths (e.g., redand/or infrared). This minimizes stray interfering light andphoto-degradation of the sensor. While, the present invention is by nomeans limited to the use of LEDs, other advantages of implementing LEDsas emitter 30 include their light weight, compactness, low powerconsumption, low voltage requirements, low heat production, reliability,ruggedness, relatively low cost, and stability. Also they can beswitched on and off very quickly, reliably, and reproducibly.

A detector 68 is provided in gas sensing portion 36 to detect radiationC. Detector 68 is positioned within gas sensing portion 36 such thatwhen gas sensing portion 3 and airway adapter 32 are coupled, detector68 receives at least a portion of luminesced electromagnetic radiation Cfrom luminescable medium 60. Based on the received radiation, detector60 generates one or more output signals related to one or moreproperties of the received radiation. For example, the one or moreoutput signals may be related to an amount of the radiation, anintensity of the radiation, a modulation of the radiation, and/or otherproperties of the radiation. In one embodiment, detector 68 includes aPIN diode. In other embodiments, other photosensitive devices areemployed as detector 68. For instance, detector 68 may take the form ofa diode array, a CCD chip, a CMOS chip, a photo-multiplier tube and/orother photosensitive devices.

Luminescable medium 60, in response to radiation B from emitter 66,emits electromagnetic radiation C in a substantially omni-directionalmanner at a wavelength different from that of the electromagneticradiation provided by the emitter. The intensity and/or persistence ofthis luminesced electromagnetic radiation rises and falls according tothe relative amounts of one or more analytes, such as oxygen, includedin the body of gas within gas flow path 50. In one embodiment, oxygencauses a modification of the intensity and/or persistence of luminescentradiation B by quenching the luminescence reaction. As the concentrationof oxygen increases, the modification of the intensity and/orpersistence of luminescent radiation B decreases. In one embodiment,luminescable medium 60 is formed as a luminescent film. For example,both of the incorporated '896 and '402 patents disclose films that maybe employed as luminescable medium 60.

Based on the output signal from gas sensing portion 36, processingportion 38 determines information related to one or more properties ofone or more analytes or constituents included in the gas disposed withinflow path 50. In the illustrated exemplary embodiment, processingportion 38 includes a processor 70 that controls emitter 66 and receivesthe signal from detector 68. Processor 70 uses the signal from detector68 to determine the oxygen concentration as discussed in detail below.Although not shown, processor 70 and/or processing portion 38 mayinclude other components typically used to monitor gas constituents,such as memory (RAM, ROM).

As shown in FIG. 3, the present invention contemplates that processingportion 38 includes an input/output device 72 or devices for providingan output of processor 70 in a human perceivable format. In an exemplaryembodiment, input/output device 72 is a monitor or display that visuallyindicates the oxygen concentration to the user. The present inventionalso contemplates that input/output device 72 includes communicationelements, such as terminals, transceivers, modems, etc. forcommunicating an output of processor 70 to a remote location. This canbe done wirelessly, via a hardwire communication system, or using anycombination thereof.

In the embodiments of FIGS. 1-3, gas sensing portion 36 and processingportion 38 are separate structures that contain their respectivecomponents. The present invention also contemplates that these twoportions can be combined into a common gas sensing/processing portion90, as shown schematically in FIG. 4 and in FIG. 5. That is, all of thecomponents necessary to detect, monitor, determine, display, andcommunicate information pertaining to the gas concentration, such as VO₂can be provided in the sensor head 90 that attaches to airway adapter32. An example of a sensor head 95 having such functionality is shown inFIG. 5 and is disclosed, for example, in U.S. patent application Ser.No. 11/368,832 (publication no. US-2006-014078-A1), the contents ofwhich are incorporated herein by reference.

The present invention contemplates that additional components can beused in gas sensing portion 36. For example, one or more filter elementscan be positioned within the gas sensing portions, e.g., betweenluminescable medium 60 and detector 68. Such filter elements 42 aretypically designed to prevent electromagnetic radiation that is notemitted by the luminescable medium from becoming incident on thedetector. For instance, in one embodiment, the filter elements arewavelength specific and permit luminescence radiation C to passtherethrough to become incident on detector 68 while substantiallyblocking radiation with other wavelengths.

Other components that can be used in gas sensing portion 36 include areference detector and a beam splitting element that directs a portionof the radiation propagating toward detector 68 onto the referencedetector. One or more output signals generated by the reference detectormay be provided to processor 70 and used as a reference to account, andcompensate, for system noise (e.g., intensity fluctuations in emitter66, etc.) in the signals generated by detector 68.

In some implementations, gas sensing portion 36 may include one or moreoptical elements (not shown) to guide, focus, and/or otherwise processradiation emitted by emitter 66 or provided to detector 68. For example,one or more lenses may collimate the radiation in a selected direction.As more particular examples, both of the incorporated '896 and '402patents disclose the use of optical elements that process radiationemitted by an emitter similar to emitter 66.

The present invention further contemplates using a thermal capacitor tomaintain luminescable medium 60 at a substantially constant operatingtemperature to reduce or eliminate inaccuracies in gas measurementsystem 30 attributable to variations in the temperature of theluminescable medium. Thus, the thermal capacitor is any device thataccomplishes this function, such as a heater controlled in a feedbackfashion based on an output of a temperature sensor, a heat sink, or thelike. Examples of suitable thermal capacitors in the form of heatingelements are disclosed in U.S. Pat. No. 6,888,101 and in U.S. patentapplication Ser. No. 11/069,114 (publication no. US-2005-0145796-A1),the contents of each of which are incorporated hereby by reference.

In the embodiment illustrated in FIGS. 1-4, a single window 62 isprovided on the airway adapter. The present invention also contemplatesproviding two windows similar to window 62 in the airway adapter. As isshown and described in the '402 patent, the two windows may be disposedin airway adapter 32 opposite from each other to enable electromagneticradiation to pass through the adapter. In this embodiment, a detector 32may be positioned on an opposite side of the airway adapter from emitter66 when sensor.

The present invention also contemplates that airway adapter 32 caninclude other one or more additional gas measuring and/or sensingcomponents. These other sensing components are schematically illustratedas 80 in FIG. 3. Examples of such sensors includes temperature, light,sound, humidity, pressure, flow, and gas concentration sensors. Suchsensors can be used to monitor the flow of gas, gas sensing portion 36or both. For example, a temperature sensor can be provided in housing 37to detect overheating in the housing. A temperature sensor can also beprovided to detect the temperature of the gas flowing in the airwayadapter.

FIG. 5 illustrates a gas monitoring system that includes both a carbondioxide (CO₂) concentration detecting capability and an oxygen (O₂)concentration detecting capability. The oxygen concentration detectingsystem corresponds to the luminescence quenching technique discussedabove and includes a luminescable material disposed on window 62 of anairway adapter 132. The CO₂ monitoring system is an absorption type gas(analyte) detection system in which energy is transmitted from anemitter (not shown) disposed on one leg of a housing 120 (such as leg122). A window 123 is shown on an interior surface of leg 122 from whichthe energy exits housing 120. The energy is provided to a first window(not shown) defined in the airway adapter. It passes through a gassample (the gas flowing through gas flow path 50), and out a secondwindow 134 also defined in the airway adapter generally opposite thefirst window. The energy exiting the sample site via second window 134is measured by a detector (not shown) provided in second leg 124.

As known in the art, the signal from the detector is used to determinethe gas (analyte) concentration. For example, it is known to use theoutput of this type of absorption system to detect the amount of CO₂ inthe gas passing through the airway adapter, which is used to determinethe amount of mixed expired CO₂ (Fē_(CO2)) and the amount of inspiredCO₂ (Fi_(CO2)). The signal from the detector can be processed by aprocessor provided in housing 37 or sent wirelessly or via a hardwire 48to a separate processing portion. In this illustrated embodiment, theprocessing portion is incorporated into housing 120 and the resultantanalyte measurement is shown on display 72.

In a similar fashion, the present invention further contemplates thatthe airway adapter can be configured to include a flow sensing system tomeasure the flow or flow rate of gas passing through the airway adapter.The flow rate is used to determine the amount of analyte passing throughthe airway adapter over a given period of time or during a respiratorycycle or phase thereof.

One type of flow sensing system suitable for use in this embodiment ofthe present invention is a pneumotach type of flow sensor. Such a flowsensor includes a flow element (not shown) that is disposed in the gasflow path so as to create a pressure drop in the flow of gas along thegas flow path. The pressure drop created by the flow element is measuredand used to determine the flow rate.

FIG. 6 illustrates an airway adapter 232 having such a flow sensingcapability. It should be noted that airway adapter also has an O₂ andCO₂ sensing capability using the techniques discussed above. Airwayadapter 232 includes a pair of ports 234 a and 234 b that are providedon each side of the flow element contained within the airway adapter.These pressure sensing elements allow the pressure drop across the flowelement to be measured so that the flow of gas through the airwayadapter can be measured quantitatively. For example, a pair of tubes orpneumatic hoses 236 a and 236 b can be coupled to ports 234 a and 234 bto and to a pressure sensor or sensors in processing portion 38 (seeFIG. 1). The pressure sensors measure the pressure drop and this outputis used to determine the flow through the airway adapter.

In the embodiment illustrated in FIG. 6, the additional flow sensingfunction is not contained in housing 37, which also contains at leastsome components of analyte sensing system. However, the presentinvention also contemplates that the flow sensing elements, such as thepressure sensor(s) and processor can be contained in housing 37. Inwhich case, ports 234 a and 234 b would be coupled directly to thehousing. In the embodiment illustrated in FIG. 6, the flow element isprovided on one side of the gas measurement site. The present inventionalso contemplates using the gas measurement site to create the pressuredrop. In which case, ports 234 a and 234 b would be provided on eitherside of the gas measurement cite. Such a configuration is taught, forexample, in the '660 patent, the '389 patent, and the '451 application.

Because mainstream oxygen sensing systems measure the oxygen in theprimary gas flow, i.e., the flow of gas in gas flow path 50, suchsystems do not include any significant gas drying or dehumidificationcomponents. As a result, the effect that temperature and humidity haveon the inspired and expired gas, namely the increase in volume of theexhaled gas, must be taken into consideration in order to obtain anaccurate oxygen consumption measurement using the Haldane transform.

To compensate for the lack of any significant gas drying ordehumidification components, the present invention determines how muchgas volume has been added due to temperature and humidificationincreases present in the expired gas and takes this into considerationin calculating VO₂. In short, the present invention, corrects the errorby correcting the inspired oxygen fraction measured using a mainstreamsensor to the same temperature and humidity conditions as the expiredgas.

The percent increase in volume due to increased temperature and humidifyis estimated by analyzing the oxygen fraction expired from the trachealdead space (Fi_(O2) _(—) _(DS)) as compared to the inspired oxygenfraction (Fi_(O2)). The tracheal dead space gas is essentially inspiredgas that has been warmed and humidified in the trachea, but has notparticipated in alveolar gas exchange. By using the oxygen concentrationas measured in the tracheal dead space gas, instead of the oxygenconcentration as measured in the inspired gas, the present inventioneffectively corrects both gases to the same conditions. The trachealdead space oxygen concentration Fi_(O2) _(—) _(DS) is substituted intothe Haldane transform and the VO₂ calculation as follows:

Vi=Ve*(1−Fē _(CO2) −Fē _(O2))/(1−Fi _(CO2) −Fi _(O2) _(—) _(DS)),and  (5)

the oxygen consumption calculation becomes:

VO ₂ =V _(E) *[Fi _(O2) _(—) _(DS)*((1−Fē _(CO2) −Fē _(O2))/(1−Fi _(CO2)−Fi _(O2) _(—) _(DS)))−Fē _(O2)].  (6)

In the event that inspired oxygen concentration is not stable orconstant during inspiration, the ratio of Fi_(O2) _(—) _(DS) to Fi_(O2)can be applied to the Fi_(O2) signal over the complete inspiration.Alternatively, the difference can between Fi_(O2) _(—) _(DS) and Fi_(O2)can be added (or subtracted) from the Fi_(O2) signal over the completeinspiration. Note that the Fi_(O2) signal used in calculating the ratio,or difference, should correspond to the gas at the end of inspirationthat remained in the tracheal dead space at the end of inspiration.

In the event that the inspired gas is heated and humidified to the samedegree as expired gas, then the ratio of dead-space F_(O2) to inspiredFi_(O2) will be 1.0. If the inspired gas is completely dry and at roomtemperature, and the expired gas is fully humidified at 37° C., then theratio will be 0.902.

To make the comparison correctly, the Fi_(O2) that is analyzed should bethe same gas that is analyzed from the tracheal dead space. This meansthat the Fi_(O2) should be calculated using the last gas to enter thelungs and corresponding to the volume of the tracheal dead space asmeasured, for example, using Fowlers method.

FIG. 7 is a graph that shows the CO₂ partial pressure plotted againstthe oxygen partial pressure during expiration only. At the beginning ofexpiration, generally indicated at 100, the oxygen partial pressurefalls while the CO₂ partial pressure remains constant. This is theeffect of heating and humidification of the tracheal gas. Thus, theoxygen partial pressure at 100 corresponds to Fi_(O2) _(—) _(DS). Thisplot corresponds to dry room temperature inhaled gas and exhaled gas atroom temperature.

An exemplary method of detecting the Fi_(O2) _(—) _(DS) value is to drawthe line using linear regression of the corresponding partial pressurevalue of the CO₂ vs. O₂ plot using only the data when CO₂ is greaterthan a threshold (i.e. 0.5% CO₂). See line 110 in FIG. 7. The pointwhere line 110 intersects the average O₂ concentration of the gas thatremained in the tracheal dead-space at the end of inspiration, line 112,is the oxygen fraction of the gas in the tracheal dead-space (Fi_(O2)_(—) _(DS)). For the breath illustrated by the plot shown in FIG. 7,this intersection occurs at point 116, so that Fi_(O2) _(—) _(DS) isapproximately 126 mm Hg.

It can also be appreciated from reviewing FIG. 7, that the Fi_(O2) atthe start of expiration, which is indicated at 114, is about 131 mm Hg.This corresponds to oxygen concentration peak of the CO₂ v. O₂ plotshown in FIG. 7. Shortly after the start of inspiration, the trachealdeadspace gas has been expelled and the gas that was exchanged in thelungs begins exits the patient. Thus, the partial pressure of oxygendrops. At the point where the CO₂ beings to increase, the exhaled gas isno longer tracheal deadspace gas. In the example shown in FIG. 7, theratio of dead-space F_(O2) to inspired Fi_(O2) (Fi_(O2) _(—)_(DS)/Fi_(O2)) is approximately 126/131=0.962. As noted above, if thisratio is “1”, there is substantially no error due to gasheating/humidification.

FIG. 8 is a graph of an exemplary oxygen concentration waveform 140 andan exemplary gas flow waveform 142 over a single breath. This figureillustrates how the oxygen gas concentration, as measured by amainstream gas monitoring system located near the patient, changesduring a typical respiratory cycle. The transition from inspiration toexpiration, which occurs when the flow rate crosses the “zero” point, isindicated by dashed line 144. During a time period 150 immediately atthe start of expiration, the oxygen concentration falls slightly becausethe gas being expelled by the patient is gas from the oral cavity, whichhas not participated in the gas exchanged in the lungs and has onlymixed slightly with the gas that has been exchanged in the lungs. Thisgas also has not been heated/humidified to any appreciable extent.During a time period 152, gas from the tracheal dead space gas, whichhas been heated/humidified is expelled from the patient. Thisheated/humidified gas, which the associated volumetric error causes ashift in the oxygen concentration measurement during this period. Theoxygen concentration at this point is the Fi_(O2) _(—) _(DS) value.

In an exemplary embodiment of the present invention, the O₂ and CO₂signals are measured as close to simultaneously as possible and aresampled at the generally the same point or location in the breathingcircuit. The present invention also contemplates that the flow signal bealigned in time with the CO₂ and O₂ signals. Selecting the gas thatremained in the tracheal dead-space requires knowledge of the volume ofthe tracheal dead-space. This volume can be determined using Fowlersmethod. Fowlers method, originally described by Fowler for nitrogen(Fowler, W. S. (1948), “Lung function studies II: the respiratorydead-space”, Am. J. Physiol., 154: 405-410.)), has been applied tocarbon dioxide by researchers, such as Fletcher et al. (Fletcher, R.,Jonson, B., Cumming, G. & Brew, J. (1981), “The Concept of DeadspaceWith Special Reference to the Single Breath Test for Carbon Dioxide,”Br. J. Anaesth., 53: 77-88). As an example, Fowlers method isimplemented in the NICO2® monitoring device sold by RESPIRONICS, Inc. ofMurrysville, Pa.

The present invention also contemplates using the techniques disclosedherein to trigger an alarm indicating that heating and or humidificationof the inspired gas is not adequate. The standard of clinical care inthe ICU is to heat and humidify the inspired gas. If gas is properlyheated and humidified, then there will be little or no differencebetween Fi_(O2) _(—) _(DS) and Fi_(O2). The presence of a significantdifference between Fi_(O2) _(—) _(DS) and Fi_(O2) may be used to triggeran alarm or message indicating a problem with the heating andhumidification of the inspired gas.

This same principle could be applied using any gas so long as the gas ispresent in significant quantity in the inspired gas and can be analyzedand aligned with the flow signal. For example, analysis of theconcentration difference between inspired and tracheal dead-spacenitrogen, nitrous oxide, helium etc., could be used to detect adifference in heat and humidification between inspired and expired gas.CO₂ could be used if it were present in the inspired gas, such as in CO₂a rebreathing maneuver.

In another embodiment, the correction can be applied to directcalculation of VO₂ (equation (1)) by substituting Fi_(O2) _(—) _(DS) forFi_(O2). Even though this correction may not be optimal, it wouldcorrect much of the error associated with gas temperature and humiditydifferences. Making this substitution in equation (1) yields thefollowing:

VO ₂ =Vi*Fi _(O2) _(—) _(DS) −Ve*Fē _(O2)  (7)

The present invention also contemplates using the techniques disclosedherein to correct the oxygram waveform. The compensation of theinspired/expired oxygen signal can be applied using knowledge of thetracheal dead-space gas. For present purposes, the expired gas isassumed to be substantially at body temperature and at 100% relativehumidity. The first gas that is expired at the mouth during every breathis gas that had remained in the trachea and did not participate in gasexchange. This “tracheal gas” is essentially inspired gas that has beenheated to body temperature and completely humidified by the trachea, buthas not had any oxygen removed by the lungs. The difference between theoxygen fraction of inspired gas and the oxygen fraction of the trachealgas corresponds to the amount of compensation that must be applied tocorrect the inspired gas to the expired gas temperature and humidityconditions.

As an example, the inspiratory portion of the oxygram may be correctedto expiratory conditions by:

1. Measurement of the difference between the oxygen concentration ofinspired gas and the oxygen concentration of tracheal gas.

2. Subtraction the difference measured in step 1 from the next, orcurrently displayed, oxygen waveform values. Also subtract the measureddifference from the inspired oxygen parameter (FiO₂). It is furthercontemplated that instead of subtracting the difference, the ratio ofwet (tracheal) to dry (inspired) oxygen concentration could be used asmultiplier of the uncorrected waveform.

3. Display the oxygen waveform with the measured expired and compensatedinspired data. It is contemplated that the flow signal can be used todetermine the exact transition time between inspired and expired gasflow.

FIG. 9 is a graph illustrating two exemplary oxygrams (oxygen percentwaveforms), corrected and non-corrected, created by mechanicallyventilating a test lung with dry room air and adding humidification tothe gas exiting the test lung. Oxygram 160 represents a waveform inwhich the inspiration of dry air and expiration of wet air is notcorrected for the differences between inspiratory and expiratory gas dueto humidification. If oxygram 160 is corrected for the differencesaccording to the principles of the present invention, oxygram 162results. It can be observed in this exemplary figure that the appliedcorrection reduces the apparent fluctuation of percent oxygen fromapproximately 1% to approximately 0.1%.

The present invention also contemplates that both the compensatedinspired and measured expired oxygen parameters (and oxygram waveform)can be adjusted to ambient conditions. Because the expired gas is at asubstantially known temperature and humidity, it can be adjustednumerically to ambient conditions prior to display. After applying thecompensation described above to the inspired data, the same numericaladjustment as was applied to the expired waveform and parameters can beapplied to the inspired oxygen data. The resulting waveform andparameters mirrors the conditions of the side-sampled waveform to whichclinicians are accustomed to visualizing.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, it is to be understood thatthe present invention contemplates that, to the extent possible, one ormore features of any embodiment can be combined with one or morefeatures of any other embodiment.

1. A mainstream gas monitoring system comprising: (a) a mainstreamairway adapter; and (b) a gas sensing assembly associated with themainstream airway adapter and comprising: (1) a gas sensing portionassociated with the mainstream airway adapter, wherein the gas sensingportion outputs a signal indicative of an analyte in a gas flow in themainstream airway adapter, and (2) a processing portion adapted toreceive the signal from the gas sensing portion, wherein the processingportion is programmed to determine an amount of the analyte in the gasflow based on the signal from the gas sensing portion, and wherein theprocessing portion is programmed to compensate for volumetricdifferences between the gas flow during inspiration and the gas flowduring expiration.
 2. The system of claim 1, wherein the processingportion compensates for the volumetric differences by monitoring atracheal dead space gas and using the monitored tracheal dead space gasto determine the amount of the analyte in the gas flow.
 3. The system ofclaim 1, wherein the gas sensing portion and the mainstream airwayadapter are configured such that the gas sensing portion is selectivelyattachable to the mainstream airway adapter.
 4. The system of claim 1,wherein a luminescable material is associated with the mainstream airwayadapter.
 5. The system of claim 1, wherein the gas sensing portionincludes: an emitter adapted to emit radiant energy; and a detectoradapted to detect energy.
 6. The system of claim 1, further comprising:a first housing containing the gas sensing portion, wherein the firsthousing is adapted to be coupled to the mainstream airway adapter; asecond housing containing the processing portion, wherein the secondhousing and the first housing are separate from one another; and acommunication link providing communication between the gas sensingportion and the processing portion.
 7. The system of claim 1, whereinthe gas sensing portion and the processing portion are contained in acommon housing that is adapted to be coupled to the mainstream airwayadapter.
 8. The system of claim 1, further comprising an output deviceadapted to provide an output of the processing portion in a humanperceivable format.
 9. The system of claim 1, wherein the analyte isoxygen, and wherein the processing portion is adapted to determineoxygen consumption (VO₂).
 10. The system of claim 9, further comprisinga flow rate determining system, and wherein the gas sensing portionincludes (a) an oxygen concentration monitoring system; and (b) a carbondioxide monitoring system
 11. The system of claim 10, wherein theprocessing portion determines VO₂ as:VO ₂ =V _(E) *[Fi _(O2) _(—) _(DS)*((1−Fē _(CO2) −Fē _(O2))/(1−Fi _(CO2)−Fi _(O2) _(—) _(DS)))−Fē _(O2)], where V_(E) is the expired volume ofgas, Fi_(O2) _(—) _(DS) is the tracheal dead space oxygen concentration,Fē_(CO2) is the expired carbon dioxide concentration, Fē_(O2) is theexpired oxygen concentration, and Fi_(CO2) is the inspired carbondioxide concentration.
 12. The system of claim 1, further comprising aflow rate determining system, and wherein the gas sensing portionincludes an oxygen concentration monitoring system.
 13. The system ofclaim 12, wherein the processing portion determines VO₂ as:VO ₂ =Vi*Fi _(O2) _(—) _(DS) −Ve*FēO ₂, where V_(I) is the inspiredvolume of gas, Fi_(O2) _(—) _(DS) is the tracheal dead space oxygenconcentration, V_(E) is the expired volume of gas, and Fē_(O2) is theexpired oxygen concentration.
 14. A method of monitoring system ananalyte in a gas flow; providing mainstream airway adapter having a gasflow therethrough; producing a signal indicative of an analyte in thegas flow; determining an amount of the analyte in the gas flow based onthe signal from the gas sensing portion; compensating for volumetricdifferences between the gas flow during inspiration and the gas flowduring expiration; and providing a signal indicative of the amount ofanalyte in a human perceivable format.
 15. The method of claim 14,wherein compensating for the volumetric differences includes monitoringa tracheal dead space gas and using the monitored tracheal dead spacegas to determine the amount of the analyte in the gas flow.
 16. Themethod of claim 14, wherein the analyte is oxygen, and furthercomprising determining oxygen consumption (VO₂) based on the amount ofanalyte.
 17. The method of claim 14, further comprising: determining aflow rate of the gas flow, and wherein determining the amount of theanalyte includes: determining the oxygen concentration in the gas flow;and determining the carbon dioxide in the gas flow.
 18. The method ofclaim 17, further comprising determining oxygen consumption (VO₂) as:VO ₂ =V _(E) *[Fi _(O2) _(—) _(DS)*((1−Fē _(CO2) −Fē _(O2))/(1−Fi _(CO2)−Fi _(O2) _(—) _(DS)))−Fē _(O2)], where V_(E) is the expired volume ofgas, Fi_(O2) _(—) _(DS) is the tracheal dead space oxygen concentration,Fē_(CO2) is the expired carbon dioxide concentration, Fē_(O2) is theexpired oxygen concentration, and Fi_(CO2) is the inspired carbondioxide concentration.
 19. The method of claim 14, wherein determiningthe amount of the analyte in the gas flow based on the signal from thegas sensing portion includes: exciting a luminescable material with anexcitation energy; and monitoring quenching of a luminescence.
 20. Themethod of claim 14, further comprising determining a flow rate of thegas flow, and wherein determining the amount of the analyte includesdetermining the oxygen concentration in the gas flow.
 21. The method ofclaim 20, further comprising determining oxygen consumption (VO₂) as:VO ₂ =Vi*Fi _(O2) _(—) _(DS) −Ve*Fē _(O2), where V_(I) is the inspiredvolume of gas, Fi_(O2) _(—) _(DS) is the tracheal dead space oxygenconcentration, V_(E) is the expired volume of gas, and Fē_(O2) is theexpired oxygen concentration.